Interpretive Summary: A project to sequence the genome of the Black-legged tick, Ixodes scapularis, has been initiated and a genome sequencing project for the Southern cattle tick, Boophilus microplus, is being considered. Information about the size and organization of the genome of these ticks is critical for making good decisions about sequencing and cloning strategies to employ in these projects. Classical reassociatiion kinetics experiments were utilized to determine the genome sizes of the I. scapularis and B. microplus were 2,100 and 7,100 million base pairs, respectively. By comparison, the human genome is approximately 3,000 million base pairs; thus, the tick genome will require significant effort and money to be completely sequenced. The organization of both tick genomes is significantly different from other arthropod genomes in that compared to other arthropods, the ticks possess a lower proportion of the unique sequence type DNA (the fraction containing most of the gene coding regions of the genome), a higher proportion of moderately repetitive type DNA, and highly repetitive type DNA of very low sequence complexity. This type of highly repetitive DNA complexity in the genome implies difficulties in assembling groups of overlapping sequences into a finalized chromosomal or genomic sequence.

Technical Abstract:
Genome sizes and the organization of repetitive DNA were determined in the hard ticks Ixodes scapularis and Boophilus microplus using reassociation kinetics. The I. scapularis genome contains ~2.15 pg (2.1 x 10*3 Mbp) of DNA and consists of no foldback (FB), 27% highly repetitive (HR), 39% moderately repetitive (MR), and 34% unique DNA. The B. microplus genome contains 7.5 pg (7.1 x 10*3 Mbp) DNA, and consists of 0.82% FB, 31% HR, 38% MR, and 30% unique DNA. In both species, repetitive sequences occur in a mixture of long and short period interspersion but most (65-80%) of the DNA follows a pattern of short period interspersion. Genome size and organization in the three tick species so far examined are distinct from other arthropods in having a greater proportion of MR, a lower proportion of unique DNA and HR DNA of very low sequence complexity.